Improve diagnostics for duplicate enum discriminants

[clarfonthey]

This is split out from the discussion in #97319.

First, the existing error for duplicate enum discriminants could be improved. The best summary of what we have now can be seen in the test added for #97319, which fixes part of it:

enum Enum {
    P = 3,
    X = 3,
    Y = 5
}

#[repr(u8)]
enum EnumOverflowRepr {
    P = 257,
    X = 513,
}

#[repr(i8)]
enum NegDisEnum {
    First = -1,
    Second = -2,
    Last,
}

Which outputs:

error[E0081]: discriminant value `3` assigned more than once
  --> $DIR/E0081.rs:1:1
   |
LL | / enum Enum {
LL | |
LL | |     P = 3,
   | |         - first assignment of `3`
LL | |
LL | |     X = 3,
   | |         - second assignment of `3`
LL | |
LL | |     Y = 5
LL | | }
   | |_^

error[E0081]: discriminant value `1` assigned more than once
  --> $DIR/E0081.rs:11:1
   |
LL | / enum EnumOverflowRepr {
LL | |
LL | |     P = 257,
   | |         --- first assignment of `1` (overflowed from `257`)
LL | |
LL | |     X = 513,
   | |         --- second assignment of `1` (overflowed from `513`)
LL | |
LL | | }
   | |_^

error[E0081]: discriminant value `-1` assigned more than once
  --> $DIR/E0081.rs:20:1
   |
LL | / enum NegDisEnum {
LL | |
LL | |     First = -1,
   | |             -- first assignment of `-1`
LL | |
LL | |     Second = -2,
   | |     ----------- assigned discriminant for `Last` was incremented from this discriminant
LL | |
LL | |     Last,
   | |     ---- second assignment of `-1`
LL | |
LL | | }
   | |_^

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0081`.

I suggested that the last version of the error should suggest explicitly stating that the discriminant for Last is computed as (Second = -2) + (1 variant later) = -1 = First, or somehow better wording that. Essentially, make it clear to the user that things are incremented positively, regardless of the order of existing discriminants.

Secondly, @Bryysen suggested that the code that does this (check_enum) could definitely use some additional refactoring as well in this comment.

[Bryysen]

Another thing we might want to do is report all the duplicates in one error, instead of generating a new error for every collision. So currently for:

#[repr(u8)]   
enum Derp {    
    First = 1, 
    Second = 1,
    Third = 1, 
    Fourth = 1,
}

We get the errors:

error[E0081]: discriminant value `1` assigned more than once
  --> tst.rs:5:1
   |
5  | / enum Derp {
6  | |     First = 1,
   | |             - first assignment of `1`
7  | |     Second = 1,
   | |              - second assignment of `1`
8  | |     Third = 1,
9  | |     Fourth = 1,
10 | | }
   | |_^

error[E0081]: discriminant value `1` assigned more than once
  --> tst.rs:5:1
   |
5  | / enum Derp {
6  | |     First = 1,
   | |             - first assignment of `1`
7  | |     Second = 1,
8  | |     Third = 1,
   | |             - second assignment of `1`
9  | |     Fourth = 1,
10 | | }
   | |_^

error[E0081]: discriminant value `1` assigned more than once
  --> tst.rs:5:1
   |
5  | / enum Derp {
6  | |     First = 1,
   | |             - first assignment of `1`
7  | |     Second = 1,
8  | |     Third = 1,
9  | |     Fourth = 1,
   | |              - second assignment of `1`
10 | | }
   | |_^

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0081`.

It would be nice if we instead got:

error[E0081]: discriminant value `1` assigned more than once
  --> tst.rs:5:1
   |
5  | / enum Derp {
6  | |     First = 1,
   | |             - 1st assignment of `1`
7  | |     Second = 1,
   | |              - 2nd assignment of `1`
8  | |     Third = 1,
   | |              - 3rd assignment of `1`
9  | |     Fourth = 1,
   | |              - 4th assignment of `1`
10 | | }
   | |_^

[rdzhaafar]

@rustbot claim

[rdzhaafar]

I don't have time to work on this, unfortunately. I did get all the errors to show up in the same error message, will paste the code here when I get to my computer.

@rustbot release-assignment

[Bryysen]

Thanks for the effort. I have a bunch of spare time going forwards so I can pick up where you left off

[rdzhaafar]

@Bryysen

Perfect! My changes are in this fork

Basically I've factored out the duplicate-checking code into a separate function in rustc_typeck/check/check.rs. I've also added a couple of derive macros to make sure that Discr<>'s can be used in a hashmap.

The code now displays all duplicates in one message. What I haven't figured out, however, is how to check that a Variant<>'s predecessor is negative, since the value is stored as a u128.

Best of luck!

[Bryysen]

@rustbot claim